Reaction and regeneration systems for use in catalytic hydrocarbon conversion processes are well known. Typical systems have at least one reactor and at least one regeneration section. In a reactor, solid catalyst particles promote certain hydrocarbon conversion reactions and at the same time deactivate somewhat. The deactivated catalyst particles are transported to the regeneration section for reactivation, and reactivated catalyst particles are transported back to the reactor.
The reactor is typically a radial flow reactor in which a reactant stream is processed in radial flow through a vertically positioned annular-form catalyst bed in an elongated chamber. Catalyst particles are maintained in a vertically positioned annular-form catalyst retaining section defined by an inner tubular-form catalyst retaining screen (generally supported by a perforated or slotted centerpipe) coaxially disposed within a vertically positioned outer tubular form catalyst retaining screen. The system may use more than one reactor, and each reactor can contain more than one annular-form catalyst bed. Catalyst can flow from one annular-form catalyst bed to the next in a series or parallel flow fashion. Multiple annular-form catalyst beds may be spaced apart from one another either horizontally (e.g., in side-by-side reactors) or vertically (e.g., in a stacked reactor). Each catalyst bed may be a moving packed bed and the particles can move by gravity flow. Illustrative of hydrocarbon conversion processes using such a reactor are catalytic reforming, catalytic dehydrogenation of paraffins, catalytic dehydrogenation of alkylaromatics, and dehydrocyclodimerization of paraffins. For more information on suitable radial flow reactors, see U.S. Pat. Nos. 3,647,680; 3,692,496; 3,864,240; 4,104,149; 4,133,743; 4,167,553; 4,325,806; 4,325,807; 4,567,023; and 5,879,537. The contents of U.S. Pat. No. 4,567,023 are incorporated herein by reference.
The reactants entering the annular-form catalyst bed have been heated and enter at an elevated temperature. The reactants in turn heat the reactor wall, the screens, and the catalyst so that at steady state the entire reactor operates at an elevated temperature. Even so, the temperature within the reactor or the annular-form catalyst bed can vary spatially, depending on whether the reactions taking place are endothermic or exothermic and on heat loss. But, as long as the temperatures are more or less constant, such temperature differences are not problematic, and are routinely accommodated for in the mechanical design of the reactor. Likewise, raising and lowering the temperatures within the reactor cause no particular difficulties with the mechanical integrity of the reactor, provided that heating and cooling is done gradually or in a controlled manner and provided, that the magnitude of the temperature change is not excessive. Then, the reactor wall, the screens, and catalyst are able to expand or contract relative to each other and according to their thermal expansion coefficients in such a way that the volume of the catalyst bed is essentially constant, the amount of catalyst in the catalyst bed is essentially constant, and no mechanical failure occurs inside the reactor.
Very rapid, uncontrolled, or non-uniform heating and cooling or a large magnitude cooling event is quite different, however. If the reactor is initially at steady state and an elevated temperature, a loss of flow of reactants can cause extreme transient differences in the volume of the catalyst bed containing a fixed amount of catalyst. The inner screen can cool sooner and faster than the wall, which can cause the bed volume to expand. This can let more catalyst enter the bed, since the bed is fed by gravity flow. Once the walls also begin to cool, however, the bed volume can contract and the bed pressure can rise, since the bed now contains more catalyst than before. This interparticle stress in turn can exert tremendous forces on the inner and outer screens, which can collapse or crack under extreme loads. A shutdown of the heater for the reactants entering the reactor can have the same effect.
A reaction and regeneration system is described in U.S. Pat. No. 3,647,680. More specifically, the figure in U.S. Pat. No. 3,647,680 shows an annular-form bed 13, a lock hopper 22, a lift engager 25, and a disengaging hopper 28. This system can be used to reduce the bed pressure and relieve the stress on the screens in the annular-form bed 13 by withdrawing catalyst particles from the bottom of the bed 13 and transporting them via lock hopper 25 and lift engager 25 to disengaging hopper 28. This system has several disadvantages, however. First, when the conditions in the reactor are changing rapidly, this system transports catalyst particles from the reactor to the regeneration section, even though it would be better for the operation of the regeneration section to transfer those catalyst particles somewhere else. Second, this system transports catalyst particles to the regeneration section even when the regeneration section is shutdown, since the same circumstances that cause rising reactor bed pressures often cause regeneration section shutdowns as well. Therefore, other reaction and regeneration systems that reduce bed pressures are sought.